. 2 moles of ideal gas at 27°C temperature is expanded reversibly from 2 lit


to 20 lit. Find entropy change. (R = 2cal/molK)

(a) 92.1

(b) 0

(c) 4

(d) 9.2

Best Answer

Correct Option is :(d) 

KEYWORD - EXPANDED  REVERSIBLY

dS = q rev/ T

From the first law of thermodynamics,

dq = dU + PdV = C dT + PdV

dividing by T

dq / T = CdT/T +PdV/T 

dS = Cv dT/T +  RdV/V

ΔS =Cv ln T2/T1 + R ln V2/V1

since T1=T2               ln T2/T1 = 0

ΔS =R ln V2/V1 =2 ln 10

= 4.605 cal/mol/K

entropy change for 2 moles of gas = 2*4.605 = 9.2cal/mol/K

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